Preheat your oven to 375 degrees F and line 2 baking sheets with parchment paper. In a mixing bowl, whisk together the flour, cornstarch, baking powder, baking soda, and salt. Set aside. In a separate mixing bowl, mix together the butter and sugar with an electric mixer until creamy. Mix in the egg and the additional egg yolk. Mix in the vanilla. Slowly mix in the flour mixture until combined. Stir in the M&M’s with a spoon, reserve 1/4 cup of M&M’s for the tops of the cookies. Scoop out 2 tbsp of dough at a time and form into a balls the place on the prepared baking sheet 2 inches apart. Bake for 10-12 minutes edges are golden. Allow the cookies to cook on the baking sheet for 5minutes then transfer to a wire rack to cool completely.